A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus each substrate to be exposed, is loaded on a substrate support on which the substrate is supported during the exposure of a patterned beam of radiation. To clamp the substrate on the substrate support a clamping device is provided. In a known lithographic apparatus a vacuum clamping device is used as a clamping device. Such a vacuum clamping device provides a vacuum force with which the substrate is clamped on the supporting surface of the substrate support. In the case a substrate is straight, the substrate will be clamped on the support surface without any substantial internal stresses in the substrate.
However, substrates may not be straight, but for instance be warped in a number of shapes, such as a corrugated shape, a cylindrical shape, a dome shape, a saddle form or another shape. This may be caused by the production method used to make the substrate, or by pre- or post exposure processes to which the substrates are subjected during the manufacture.
When a warped substrate, for instance a dome-shaped substrate is clamped on a substrate support for instance by means of a vacuum clamp, the substrate may first contact with the substrate support at the outer circumference of the substrate and thereafter over the rest of the surface of the substrate. Due to the clamping force the substrate is forced into a substantially straight form, while the actual clamping starts at the outer circumference of the substrate. As a result stresses may be induced in the substrate when it is clamped on the supporting surface.
These stresses may have a negative influence on the quality of the integrated circuits. Also, since the substrate is clamped in another form than desired, the overlay performance of the projections of the lithographic apparatus may decrease which may have a negative influence on the quality of the integrated circuits.
It is remarked that US 2009/0086187 A1 discloses a substrate support comprising a clamping device configured to clamp a substrate on a substrate support location, wherein the clamping device comprises a first device to exert an attracting force on the substrate and a second device to exert a rejecting force on the substrate. This combination of a simultaneous attracting force and rejecting force should shape the substrate to a desired shape before clamping the substrate on the substrate support location.